Legal classification of the project: Epidemiological study. Number of subjects required: The inclusion of 150 cases of meningitis per year would allow the collection of about 450 cases over a period of 3 years. For a mortality rate of 20%, we would be able to estimate this percentage with a confidence interval of ± 3.7% (95% confidence interval: 16.3-23.7). With 90 deaths, a power of 80% and a level of significance of 5% (two-tailed), this sample size would demonstrate relative risks of 1.8 for a prevalence of the risk factor of 25% and 2.6 for a prevalence of the risk factor of 10%.